[`pep8-naming`] Consider any number of leading underscore for `N801` (#15988)

## Summary

The PR addresses the issue #15939 

Let me know if you think there are other test cases I should add ;-)

crates/ruff_linter/resources/test/fixtures/pep8_naming/N801.py

@@ -6,6 +6,10 @@ class _bad:
     pass
 
 
+class __bad:
+    pass
+
+
 class bad_class:
     pass
 
@@ -13,6 +17,8 @@ class bad_class:
 class Bad_Class:
     pass
 
+class Bad__Class:
+    pass
 
 class BAD_CLASS:
     pass
@@ -32,3 +38,6 @@ class GoodClass:
 
 class GOOD:
     pass
+
+class __GoodClass:
+    pass

crates/ruff_linter/src/rules/pep8_naming/rules/invalid_class_name.rs

@@ -54,7 +54,7 @@ pub(crate) fn invalid_class_name(
     name: &str,
     ignore_names: &IgnoreNames,
 ) -> Option<Diagnostic> {
-    let stripped = name.strip_prefix('_').unwrap_or(name);
+    let stripped = name.trim_start_matches('_');
     if !stripped.chars().next().is_some_and(char::is_uppercase) || stripped.contains('_') {
         // Ignore any explicitly-allowed names.
         if ignore_names.matches(name) {

crates/ruff_linter/src/rules/pep8_naming/snapshots/ruff_linter__rules__pep8_naming__tests__N801_N801.py.snap

@@ -1,6 +1,5 @@
 ---
 source: crates/ruff_linter/src/rules/pep8_naming/mod.rs
-snapshot_kind: text
 ---
 N801.py:1:7: N801 Class name `bad` should use CapWords convention
   |
@@ -16,23 +15,41 @@ N801.py:5:7: N801 Class name `_bad` should use CapWords convention
 6 |     pass
   |
 
-N801.py:9:7: N801 Class name `bad_class` should use CapWords convention
+N801.py:9:7: N801 Class name `__bad` should use CapWords convention
    |
- 9 | class bad_class:
+ 9 | class __bad:
-   |       ^^^^^^^^^ N801
+   |       ^^^^^ N801
 10 |     pass
    |
 
-N801.py:13:7: N801 Class name `Bad_Class` should use CapWords convention
+N801.py:13:7: N801 Class name `bad_class` should use CapWords convention
    |
-13 | class Bad_Class:
+13 | class bad_class:
    |       ^^^^^^^^^ N801
 14 |     pass
    |
 
-N801.py:17:7: N801 Class name `BAD_CLASS` should use CapWords convention
+N801.py:17:7: N801 Class name `Bad_Class` should use CapWords convention
    |
-17 | class BAD_CLASS:
+17 | class Bad_Class:
    |       ^^^^^^^^^ N801
 18 |     pass
    |
+
+N801.py:20:7: N801 Class name `Bad__Class` should use CapWords convention
+   |
+18 |     pass
+19 |
+20 | class Bad__Class:
+   |       ^^^^^^^^^^ N801
+21 |     pass
+   |
+
+N801.py:23:7: N801 Class name `BAD_CLASS` should use CapWords convention
+   |
+21 |     pass
+22 |
+23 | class BAD_CLASS:
+   |       ^^^^^^^^^ N801
+24 |     pass
+   |